"Garry's Mod" (GMod) is a sandbox, physics game based on Valve's Source engine, allowing players to construct physics based objects like vehicles and traps. It can be played in single player mode or online, cooperatively or competitively. This style of game is more like a toy, in the sense that there are no concrete goals other than on what players agree is the goal. However, with many Source-based games, like "Half Life 2" and "Left 4 Dead," the single player campaigns can be played through, with all of the advantages of GMod's sandbox tools.
Download Steam, a gaming platform, then run and install it by double clicking on the installer *.msi file. Log onto Steam and download "Half-Life 2" and "Garry's Mod."
Open the Steam Client, select "Games," then double-click on "Garry's Mod." Click on "Play Singleplayer" on the Main Menu.
Click on the "Icons" tab, then scroll down to "Half Life 2" and click on the "+" symbol to expand the maps for "Half Life 2." Double-click on the first map to start the "Half-Life 2" campaign. Once completing a section or a level, the next level in sequence will load, as normal.